VUV range spectroscopy is one of the most efficient methods for plasma diagnostics applied in studies of hot fusion plasma, active medium of X-ray lasers, and astrophysical plasma. Extreme ultraviolet (EUV) range spectroscopy is relevant in terms of developing lithographic EUV radiation sources based on high-temperature plasma for mass production of new generation ICs.
For VUV range, spectrometers which employ phase reflection diffraction gratings with beam grazing incidence, find use. Generally, concave grooved glass diffraction gratings with gold or tungsten coatings, are applied. Spherical gratings are simpler as compared to toroidal gratings which help increase luminosity and improve spatial resolution. The grazing angle is 1-10°.
A soft X-ray and VUV range spectrometer is known, which comprises the entrance slit, the concave phase grazing incidence diffraction grating and the detector, located on the Rowland circle, Schwob J. L., et al. Review of Scientific Instruments, 58, 1601 (1987). The spectrometer is designed with an arrangement similar to that of Rowland. The difference consists in the registration surface being flat and not concave along the Rowland's circle, as it is required to ensure a wide spectral recording region. The spectrometer is characterized by a sufficiently high spectral resolution. However, due to limited recorded spectral region, several spectrometers were required in the above-mentioned paper, making the measurement procedure much more complicated. Besides, for certain detectors, such as those using microchannel plates (MCP), their illumination with grazing incidence results in significant reduction of detector quantum efficiency, and at the same time, dramatically increases the level of noise.
These drawbacks are partially overcome in the spectrometer with an entrance slit, an amplitude slit grating, an optical detector based on digital backlit CCD camera designed for recording in the 1st or −1st orders of VUV radiation spectra, Wilhein, T., et al. Review of Scientific Instruments, 70(3), 1694 (1999). A key feature of the spectrometer consists in using an amplitude grating with the 1:1 ratio of transmissive and non-transmissive parts which ensures suppression of all even spectrum orders, as well as significantly reduces radiation intensities in the odd higher diffraction orders. The spectrometer is characterized by a wide spectral range, high luminosity and spectral resolution, as well as small size. Amplitude grating efficiency can be calculated theoretically, which allows taking absolute measurements in a fairly wide spectral range, if an absolutely calibrated optical detector is used.
However, the transmissive grating is expensive, fragile and subject to contaminant deposition. With time, this can result in deterioration of grating structure and quality of recorded spectral image, reducing the instrument's reliability. Besides, serious challenges exist in the long wavelength range (λ≥100 nm) in terms of calculating the slit grating transmission. It becomes indefinite, which limits the spectral range for absolute measurements.
These shortcomings are not present in the compact soft X-ray and VUV range spectrometer designed with the configuration based on “outside Rowland” spectrum recording, Shevelko A. P. et al. Plasma Physics Reports. 34 (11), 944 (2008). The spectrometer comprises an entrance slit located on the Rowland's circle, a concave grazing incidence diffraction grating, and an image detector, whose input surface crosses the Rowland's circle at an angle which is close to normal angle. Notably, spectrum is recorded in the plane that is perpendicular to diffracted beams which provides for comfortable and easy application of spectrometer with various detector types. Spectrometer benefits include its small size, ease of operation and excellent reliability.
However, in the above-mentioned geometry of “outside Rowland's” spectrometer, ideal focusing of diffracted radiation occurs only in one point—point of registration plane intersection with the Rowland's circle, while moving away from this point results in decrease of spectral resolving power due to defocusing. As a result, the recorded spectral range is quite narrow, and the spectral resolution is fairly low.
This shortcoming is partially overcome in the compact VUV spectrometer with concave grazing incidence diffraction grating where the entrance slit is located inside the Rowland's circle, and the optical detector surface intersects twice the focusing surface of spectral lines which corresponds to position of the entrance slit removed from the Rowland's circle, Antsiferov P. S., et al. Review of Scientific Instruments 87, 053106 (2016). The spectrometer ensures a wider range of recorded VUV spectrum, with the long-wavelength limit reaching around 50 nm.
However, for certain applications measurements in a wider VUV spectrum range are required. For example, when radiation of lithographic EUV sources is recorded, it is preferable to take measurements both near the source working wavelength, λ=13.5 nm, and in the widest possible VUV range with the capability of measuring absolute radiation yield in various spectrum ranges.